Most fire detection technology for detecting the presence of a flame of burning hydrocarbon fuel focuses on detecting heat, smoke (particle matter) or flame (light), i.e., the three major characteristics of fire. All of these characteristics also have benign sources other than fire, such as heat from steam pipes, particle matter from aerosols, and light from the sun. Other factors further confound the process of fire detection by masking the characteristic of interest, such as air temperature, and air movement. In addition, smoke and heat from fires can dissipate too rapidly or accumulate too slowly for effective detection. In contrast, because flame sensors are optical devices, they can respond to flames in less than a second.
In an exemplary application, the flame of burning hydrocarbon fuel is the augmentor flame (afterburner pilot flame) in a gas turbine engine, the loss of which requires an automatic engine control to prevent fuel flow to the afterburner; otherwise, dangerous fuel levels can accumulate within seconds of the turbine engine losing its ignition flame. Failure can result in an overpressure condition leading to engine damage. In this scenario, an optical flame sensor is adapted to transmit a flameout condition, which quickly alerts the engine control system to make critical adjustments, e.g., adjusting fuel flow to prevent a potentially catastrophic situation.
Optical flame sensors can detect infrared (IR), Ultraviolet (UV), or a combination of UV and IR radiation. A UV flame sensor typically detects radiation emitted in the 200 to 400 nm range. Optical sensing devices incorporating a UV detector to sense the presence of the augmentor flame in gas turbine engines sense UV radiation emitted from the augmentor flame against the background of hot metal, in a high temperature environment and under heavy vibration.
Disadvantages of UV flame sensors known in the art include the presence of leakage currents and/or parasitic capacitances for a necessary feedback capacitance in the pre-amplifier stage coupled with an inability to survive high temperature operating conditions. For example, not all capacitors that feature a low dissipation factor are rated for high temperature environments. Also, high temperatures can cause preamplifier input offset errors. Moreover, many UV flame sensors suffer from an insufficient sensitivity to low levels of UV light, due to losses at various components, e.g., the Printed Circuit Board (PCB), feedback capacitor, and amplifier inputs.